1. Field of the Invention
The present invention is related to an orthopedic medical device. In particular, the present invention is related to a unicondylar knee implant system.
2. Description of the Related Art
Orthopedic knee implant systems have been used for many years to treat patients with knee joints that have been damaged by trauma or disease, such as osteoarthritis, rhumetoid arthritis, and avascular neurosis. A knee arthroplasty resects, cuts, or resurfaces the damaged sections of the knee and replaces them with an endoprosthetic or implant.
Most knee implant systems are tricompartmental implants and the surgical procedure used with tricompartmental implants is commonly known as total knee arthroplasty. These implants are known as tricompartmental implants because they are used when the femur is prepared to receive an implant by resurfacing or resecting the three compartments of the distal femur, i.e., the medial and lateral condyles and the trochlear groove. Regardless of the type of implant used, all arthroplasties require the bone to be specifically prepared to receive a corresponding implant by resecting, resurfacing, or deforming the bone to accept the implant.
Most knee implant systems are modular systems with multiple sizes and thicknesses and degrees of interchangeability between the sizes. For example, a typical total knee implant system may consist of 6 femoral implant sizes, 6 tibial tray implant sizes, and 6 tibial bearing implant sizes with each tibial bearing size having 7 different thicknesses. The various implant sizes are required to meet the various size bones of patients. Typical implant systems often allow interchangeability between sizes for example; a medium size femoral implant may be used with a single larger size tibial implant and tibial bearing implant to match the specific needs of a patient. However, most knee implant systems are limited with regards to the range of interchangeability between sizes; for example, the largest size femoral implant component typically cannot be used and is not compatible with the smallest size tibial tray and tibial bearing component. This is because such a high degree of mismatch between sizes often affects the range of constraint designed into the implants. Moreover, conventional implant systems require a prepared bone surface to be further resected or resurfaced whenever a different size implant is required, adding to the overall surgical time, complexity of the surgery, and bone loss.
Tibial tray implants typically have some sort of keel associated with the tibial tray to provide a level of initial fixation. Other methods if initial fixation have also been used with tibial trays, such as spikes protruding from the bone engaging surface of the implant or screws inserted through the tibial tray into the bone. However, the use of screws has been associated with osteolysis and provides a pathway for debris to enter the tibial bearing tibial tray interface. Typical keel designs have simply been of a straight cylindrical or vertical plane geometry extending from the bone engaging surface of the tibial tray implant. Accordingly, these tibial tray keel designs provide minimal initial fixation and require a significant amount of exposure of the knee joint to implant as these designs are implanted with a top down approach. A top down approach is defined for example, as implanting the tibial tray into a prepared horizontal tibial surface when the tibia is set in a vertical orientation.
Minimally invasive surgery (“MIS”) has become of great interest within the field of orthopedics. Thus, unicondylar or unicompartmental knee implants have become of great interest in the orthopedic industry due to their small size and applicability to MIS surgical approaches. Unicondylar knee implants are designed to replace only a single condyle (e.g., the medial or lateral condyle) of the distal femur.
Minimally invasive knee surgery has not yet been fully defined. However, minimally invasive knee surgery has generally been considered to include a smaller incision. A typical incision length for a total knee replacement can be up to 10 to 12 inches long. The general theory behind MIS is that with a smaller incision length, the patient will be able to recover from surgery faster.
Generally, the clinical outcomes for unicondylar knee implants have varied. Studies have reported long term survival rates for unicondylar implants to be less than that of comparable total knee implants. One particular cause for such discrepancies is due to technique associated with the implant.
The unicompartmental implant most studied is the Oxford implant. The Oxford implant is a mobile bearing unicompartmental implant. Moreover, the Oxford implant is implanted with a free-hand technique, i.e., where the bone resections are not guided by instrumentation. Thus, the clinical outcomes for these implants have varied depending upon the particular surgeon implanting the device. Accordingly, a surgeon proficient in this technique is more likely to have a good outcome whereas a less experienced surgeon is more likely not to have as good an outcome with this implant.
Recent advancements in unicondylar knee implants have resulted in techniques that employ instrumented techniques for implantation. U.S. Pat. No. 6,554,838 to McGovern et al. discloses a unicondylar knee implant that uses a guided burring technique to implant the femoral component. However, instrumentation for such implant designs are bulky and required to be operated from various angles as opposed to a predominantly anterior orientation. Accordingly, such instrument designs are not completely conducive to minimally invasive surgical approaches or a reproducible surgical result.
U.S. Pat. No. 5,326,361 to Hollister discusses an alternative approach to knee biomechanics that views the knee to move through a range of motion from flexion to extension about a single axis of rotation. This view is a departure from the traditional view that the knee profile is defined by multiple centers of curvature and thus multiple axis of rotation. In particular U.S. Pat. No. 5,326,361 discloses a total knee implant system having a single cross-sectional sweep from zero to over 120 degrees.
The development of orthopedic implant designs has been moving towards meeting the requirements of high demand patients. Patients nowadays are requiring more from their implants and since patients are living longer these days, are requiring implants to last longer. Accordingly, developments have been made in materials used to make orthopedic implants to improve implant survival rates, such as the use of cobalt chromium alloys, titanium alloys, diamond-like coatings, ceramics, hydroxyapatite coatings, and ultra-high molecular weight polyethylene materials. Moreover, implant designs have changed to meet the ever increasing demands of patients, for example the general population of Asian countries prefers implants that allow for a higher degree of flexion than traditional implants.
The relationship between flexion angle and femoral-tibial conformity is determined by gait analysis studies. For example, gait data for Caucasian population is presented in Harrington I. J., et al. “Static and Dynamic Loading Patterns in Knee Joints with Deformities”, J.B.J.S., Vol. 65-A, No. 2, February 1983, pp. 247-259. Based upon such studies the relationship between flexion angle and femoral-tibial conformity varies for different genders and races.
Thus, there is a need for a unicompartmental knee implant system that addresses the above mentioned deficiencies in current implant designs while simultaneously being suitable for minimally invasive surgical techniques.